Abstract ATM-9709684 Clark, Peter U., and Glasmann, J. R. Oregon State University Origin of the Mid-Pleistocene Transition This project is a collaborative effort that will test a hypothesis for the origin of the mid-Pleistocene spectral change seen in the (18O record at (0.8 Ma. The central issue addresses two problems in explaining global ice volume variations driven solely by changes in insolation: (1) a transition, in the middle Pleistocene, from low-amplitude, higher frequency (41 ka) to high-amplitude, lower frequency (100 ka) ice volume variations under essentially the same orbital forcing; and (2) the dominance of the 100-ka ice volume signal since the middle Pleistocene when the insolation forcing at that period (eccentricity) has essentially no power. This award will support a test of the hypothesis that a significant change in the basal boundary condition of the Laurentide Ice Sheet occurred during the middle Pleistocene, resulting in a transition from an areally extensive, but thin, ice sheet underlain by soft, easily deformable sediment to a similarly extensive, but thicker, ice sheet underlain by a mixture of soft beds and hard bedrock. The thinner ice sheets responded in a fundamentally different manner to the same climate (insolation) forcing than the thicker ice sheets, giving rise to the observed response of predominantly 41-ka cycles prior to the mid-Pleistocene transition, and predominantly 100-ka cycles thereafter. The mineralogy of these sediments will be examined for a change suggesting a source from highly weathered and altered regolith (soft beds) to fresh crystalline bedrock (hard bed). An array of models will be used to test the hypothesis that a change in the basal boundary condition gives rise to a fundamentally different response of ice sheets to the same climate forcing. Support for this hypothesis will be significant in understanding the role of ice sheets in the climate system.
